1. Field of the Invention
The present invention relates to a bolt fastening structure for an internal combustion engine, and to an engine incorporating same. More particularly, the present invention relates to a bolt fastening structure which is assembled by engaging an external thread formed at a tip portion of a bolt extending through a cylinder of the engine with an internal thread formed on an end face of a crankcase adjacent the cylinder.
2. Description of the Background Art
There is a known internal combustion engine in which stress is not concentrated on a fastening portion of the engine at a location to which a high load is applied by a pressure of combustion gas. An example of such internal combustion engine is disclosed in the Japanese Patent Laid-Open No. 2002-339931.
In the internal combustion engine, as disclosed in the Japanese Patent Laid-Open No. 2002-339931, a bolt fastening structure for a crankshaft rotational supporting portion for supporting a crankshaft for rotation by means of a crankcase and a main bearing cap is discussed.
In the bolt fastening structure, as disclosed the Japanese Patent Laid-Open No. 2002-339931, a cavity portion is formed continuously to an interior portion of an internal thread formed in the crankcase of the internal combustion engine.
A position of the tip of an effective thread portion of a bolt screwed in the internal thread is set between the most interior end of the internal thread portion, and a position displaced by two pitches to the base end side of the bolt from the most interior end such that the stress concentration in the vicinity of the internal thread portion is moderated by the setting of the tip position of the effective thread portion of the bolt.
However, a length of the internal thread portion from the bolt fitting end of the internal thread portion to the cavity portion, and a length from an abutting face of a head portion of the bolt with the crankcase to the tip of the effective thread portion of the bolt must be appropriately managed to minimize problems associated with stress concentration. The cited disclosure appears to have problems in managing such lengths. Therefore, there may be a fault in productivity.
The present invention has been made to overcome such drawbacks of the existing bolt fastening structure for an internal combustion engine. Accordingly, it is one of the objects of the present invention to provide a firm bolt fastening structure for an internal combustion engine.